"Learning note", "C language" macro definition

1. Macro definitions can be divided into 2 types:
Macro definition with no parameters
Macro definition with Parameters

2. Definition

General form
#define Macro Name string
Like #define ABC 10.
The string on the right can also be omitted, such as # define ABC

3. Role

It is used to define constants by replacing all "macro names" in the source program with the right "strings" when compiling preprocessing.
Then write a program that calculates the perimeter based on the radius of the circle

4. Example

A macro called PI is defined in line 4th, and after the pre-processing of the compilation, the 2 * PI *radius in line 8th becomes 2 * 3.14 * radius.

1#include <stdio.h>2   3   //all the macro names in the source program will be replaced by 3.14 when compiling the preprocessing.4   #definePI 3.145 6  //Radius calculation perimeter based on radius of Circle7  floatGirth (floatradius) {8     return 2* PI *radius;9 }Ten   One intMain () A  { - floatg = Girth (2); -      theprintf"perimeter is:%f", g); - return 0; -}

5. Note

1> macro names are generally capitalized in order to distinguish them from variable names, but there is no syntax error in lowercase

2> the characters within the string that are enclosed in double quotation marks for the program, and does not replace the macro. Like what:

1 #define R2  int  main ()3  {4      char" Radio " ; 5      return 0 ; 6  }

A macro called R is defined in line 1th, but ' R ' in line 4th is not replaced with 10. Radio

3> does not make a syntax check when it replaces a macro name with a string in the compilation preprocessing, but simply replaces the string. The source program that has expanded the macro name is checked for syntax only at compile time

1 #define I2  int  main ()3  {4      int i[3] = I; 5      return 0 ; 6  }

The valid range of 4> macro names is from the defined position to the end of the file. If you need to terminate the scope of a macro definition, you can use the #undef command
#define PI 3.14
/*
.
.
.
.
*/
#undef PI
PI This macro is valid between lines 1th through 8th and is invalid after line 8th.

5> you can refer to a macro name that you have defined when defining a macro
#define R 3.0
#define PI 3.14
#define L 2*pi*r
#define S Pi*r*r

6. Code

1 /*2 1. All pre-processing instructions are preceded by #3 2. Pre-processing instructions are divided into 3 types4 1> Macro Definition5 2> Conditional Compilation6 The 3> file contains7 3. Pre-processing instructions are executed before the code is translated into 0 and 18 4. The location of the preprocessing is casually written9 5. Scope of preprocessing directives: starting from the line where the instruction was written, until the end of the file, you can use #undef to cancel the function of the macro definitionTen 6. Macro names usually start with uppercase or K, and variable names are usually lowercase One  */ A#include <stdio.h> -  -  the //#define KCOUNT 4 -  - intMain () - { +     Char*name ="COUNT"; -      +printf"%s\n", name); A      at     #defineCOUNT 4 -      -     intAges[count] = {1,2, the, the}; -      -      -      in      for(inti =0; i<count; i++) { -printf"%d\n", Ages[i]); to     } +      -     //from this line, count this macro will expire the #undefCOUNT *      $     intA =COUNT;Panax Notoginseng      -     return 0; the } +  A voidTest () the { +      -}

1 /*2 1. The macro with parameters is defined as a higher efficiency than the function3  4  */5 6 /*7 int sum (int a, int b)8 {9 return a + B;Ten }*/ One#include <stdio.h> A  - #defineSum (v1, v2) ((v1) + (v2)) -  the #definePingfang (a) ((a) * (a)) -  - intMain () - { +     //Pingfang (5+5) (10*10) -     //Pingfang (5+5) +     //Pingfang (5+5) (+) A     //Pingfang (5+5)/pingfang (2) at     intc = Pingfang (5+5)/pingfang (2); -      -printf"C is%d\n", c); -     /* - int c = SUM (2, 3) * SUM (6, 4); -      in printf ("C is%d\n", c);*/ -     /* to int a = ten; +      - int b =; the      *      $ int c = SUM (A, b);Panax Notoginseng      - printf ("C is%d\n", c); the //int c = SUM (A, b);*/ +      A     return 0; the}

"Learning note", "C language" macro definition